1. Field of the Invention
The present invention relates to a memory device having an uneven pattern.
2. Description of the Related Art
With a recent development of an information-oriented society, the amount of data processed in computers increases more and more. To meet the demand under the circumstances, various types of large-capacity memories such as 16 M bit DRAMs or optical disc memories have been developed. Also, it is required that the memories be accessed at high speed for high-speed data processing.
In order to increase the memory capacity of a recording medium, it suffices if the size of the recording medium is increased. However, if the size of the recording medium increases, there occur electrical problems such as an increase in parasitic capacitance or parasitic inductance, and mechanical problems such as an increase in range of operation. Consequently, the access speed of the memory decreases. Under the circumstances, the reduction in size of the memory has been developed for attaining the high-speed memory access.
For example, the access speed of the memory using electric circuits is increased by integrating the circuits on a semiconductor substrate. Also, the high-speed memory access of an optical disc memory is achieved by reducing the size of a data record region (memory pit) and increasing the density of memory pits. However, the reduction in size of the memory and the increase in access speed by means of these techniques are close to the limits.
In general, in the memory using electric circuits, a lithographic method is used to form a design pattern on a semiconductor substrate. In this method, the finer the design pattern becomes, the less ignorable the interference of light (electromagnetic wave) radiated from a light source becomes. As a result, the reduction of the width of wiring lines is limited. On the other hand, in the optical disc memory, memory pits are formed, for example, by radiating a laser beam with a small diameter onto a material, thereby forming pits with an uneven configuration or changing physical properties such as reflectivity or refractive index. In this case, too, the reduction of the diameter of a beam is limited by the interference of light (laser beam), and accordingly the reduction of the size of each memory pit is limited.
A scanning tunneling microscope (STM) is known as a surface observation device with high resolution. When a pointed tip of a metal probe is approached to the surface of a workpiece at a distance of about 1 nm, and a voltage is applied across the probe and the workpiece, electrons are allowed to flow through a gap (tunnel effect), which was considered impossible from the view-point of classical mechanics, and a tunnel current flows therebetween. The ST takes advantage of this tunnel effect. The probe is moved in three-dimensional directions while detecting the tunnel current to observe the surface configuration of the workpiece. The resolution of the STM is about 0.1 nm, and the atomic arrangement of the surface of the workpiece can be observed. It has been proposed that a memory be manufactured according to the principle of the STM with high resolution.
U.S. Pat. No. 4,575,822 (to Quate) discloses a method and an apparatus for recording data, wherein a voltage is applied across an electrically conductive probe and a substrate capable of holding electric charge, and perturbation is caused in the substrate by a tunnel current flowing therebetween. Data is recorded in accordance with the presence/absence of the perturbation. Since this method utilizes the variation in work function of the substrate due to an electric field, the size of a memory bit is considerably greater than that of an atom. For example, in the case of a substrate with a capacitance of 1 mF per 1 cm.sup.2, the density of charge becomes 10.sup.-8 /.ANG..sup.2, when the potential of charge in the substrate is set to 10 mV or more to avoid thermal disturbance. In other words, the area affected by an electric field of one electron, i.e. the size of a memory pit, is 10.sup.8 .ANG..sup.2. Thus, the atomic-level resolution of the STM is not fully utilized.